Spatially resolving the 11.3mic. UIR band in Herbig Ae/Be Stars

Coordinator: A.Carmona, M.E.van den Ancker

Abstract:We propose to obtain LR spectroscopy of five new nearby (d 250 pc) Herbig Ae/Be stars at 11.4 micron. The detection of Unidentified InfraRed (UIR) emission bands at 8.6, 11.3, and 12.7 micron in spatially resolved observations will permit us to determine the disk geometry of these systems. Given that 10 micron emission bands could be only produced by warm dust, small superheated grains or by PAH's, the detection of extended emission will strongly suggest that it originates in the outer parts of a flared disk. The study of new nearby Herbig Ae/Be stars is essential to obtain constraints on the evolutionary models of protoplanetary disks and therefore in the scenarios of formation of planetary systems.

The recent discovery of more than one hundred Extrasolar Planets, combined with the increasing amount of studies of young stellar objects strongly suggest that the Formation of Planetary Systems is a phenomenon closely linked to the star formation process itself. In the context of the actual paradigm of planet formation, planets form out of a circumstellar disk composed of gas and dust in a time scale of a few million years. Although present theoretical and observational evidence is consistent with this scenario, the amount of nearby stars that have been spatially resolved and carefully studied is too few for determining a well constrained evolutionary time line for the planet formation process, and its dependence on the star's mass. Fundamental questions like: what is the geometry of protoplanetary disks? how does it evolve in time? are still poorly constrained with the observational information until now acquired. In order to obtain an accurate view of the mechanisms involved in planet formation it is fundamental to identify and study in detail a larger sample of spatially resolved nearby Young Stellar Objects (YSO).This research proposal is focused on the study of a new sample of nearby HerbigAe/Be stars (d250pc). Herbig Ae/Be stars (HAEBES) are intermediate mass YSO and are important because they posses a circumstellar disk where the process of planet formation is probably taking place. HAEBE stars are suitable for studies at high spatial and spectral resolution because they are nearby and bright. Studies of dust emission in HAEBES (Meeus et al 2001) suggest that according to their SED their disks could be classified as flared (group I) or self shadowed (group II). In a flared disk the vertical height is larger at larger radial distances from the central star. Theoretical models (Dullemond et al 2004) suggest that disks evolve from a flared geometry to a flat geometry. To test this, research in new samples of spatially resolved nearby HAEBES is necessary. The few HAEBES that have already been spatially resolved do not provide enough information for constrain the disk evolution time line. Previous studies using TIMMI2 (van Boekel et al 2004) demonstrated that it is possible to spatially resolve the Unidentified InfraRed (UIR) band emission. The technique used was to compare the FWHM of the observed PSF with the FWHM of the expected PSF of a point source. However, given the limitations on spatial resolution Geers et al (2004) were not able to reproduce those results using a similar set up. With VISIR at the VLT the limitation on spatial resolution will be overcome.In order to determine disk vertical geometry mid-IR spectroscopic observations are particularly suitable. Transiently heated small dust grains produce the UIR emission bands at 8.6, 11.3, and 12.7 micron. In a flat disk these emission bands are expected to come from an disk's area close to the star. However, in a flared disk, this band emission will be also produced in the outer part. Mid-IR LR long slit spectroscopy with VISIR is able to spatially resolve these bands. If extended IUR emission is detected, we will have a strong reason to believe that the observed stars have flared disks.

Observing strategyWe propose to do long slit LR spectroscopic measurements at 11.4 micron. All our targets are bright, but since we want to spatially resolve the 11.3 micron UIR band emission that is also a requirement. PAH luminosities in HAEBES ranges between 0.1 and 10 Lsun, assuming a typical PAH spectra, this would translate to 11.3 micron line fluxes of 10-15 to 10-13 W/m2 for stars within 250 pc (Acke & van den Ancker 2004). Taking these line fluxes as type, in 20 min of integration with VISIR, a 10 micron detection of a PAH 11.3 micron flux of few times 10-16 W/m2 is possible. Therefore we adopt a integration time of 30 min per target. Taking into account overheads a total amount of 56 min would be required for one science observation. For obtaining a good quality spectra calibration and sky subtraction, a total of 3 spectro-photometric standards (STD) at a range of airmass are necessary each night. The typical exposure time per STD is of about 10 minutes. Taking into account overheads, 30 minutes should be dedicated for each STD. Summing all the time needed for science and calibration observations, our program would require in total 5 hours.